1. Field of the Invention
The present invention relates to processes for producing an aromatic polycarbonate oligomer and an aromatic polycarbonate.
2. Description of the Related Art
Heretofore, there have been known methods for producing continuously an aromatic polycarbonate oligomer or an aromatic polycarbonate by reacting an aqueous alkali metal base or alkaline earth metal base solution of an aromatic dihydroxy compound with a carbonyl halide compound.
For example, as a continuous method for producing an aromatic polycarbonate oligomer, there are known methods disclosed in Japanese Patent Publication Nos. Sho 46-21460 (U.S. Pat. No. 3,646,102) and Sho 56-44091 (U.S. Pat. No. 4,089,888). These methods do not use any polymerization catalyst and molecular weight controlling agent.
These methods use a tubular reactor, and are useful when such type of reactor is available. However, it is often preferable to use a more conventional apparatus for various purposes such as a tank reactor with a stirrer and the like.
In addition, when a tubular reactor is used, an organic solvent is vaporized in the tube at a high reaction temperature and cavitation, clogging and the like occur, and therefore, stricter cooling is often required than in the case of a tank reactor.
Further, in the above-mentioned methods for producing an aromatic polycarbonate oligomer, the haloformation reaction (production of an aromatic polycarbonate oligomer) is carried out in the absence of a molecular weight controlling. Therefore, a molecular weight controlling agent is added to the aromatic polycarbonate oligomer thus produced and then a polycondensation reaction is effected to produce an aromatic polycarbonate. However, this method has the disadvantage in that the rate of end capping of the resulting aromatic polycarbonate is very low.
Aromatic polycarbonates are thermoplastic polymers, and with respect to recycling of plastics to which people have recently paid attention, aromatic polycarbonates are advantageous since the recycling can be easily effected.
However, it is known that in the case of an aromatic polycarbonate having a low rate of end capping, rearrangement of the high polymer chain occurs when melted and the molecular weight and the molecular weight distribution change [Kobunshi Kagaku, 21, 505 (1964); J. Polymer Sci., 55, 251 (1961)].
It is well known to those skilled in the art that molecular weight and molecular weight distribution most closely relate to physical proporties of polymers, and when such phenomena as described above occur, physical properties of the aromatic polycarbonates disadvantageously change or deteriorate when performing melt-shaping during recycling of the aromatic polycarbonates.
Therefore, a method for producing aromatic polycarbonates having a very high rate of end capping and a high polymer chain which is hardly rearranged is strongly desired.
It is known that aromatic polycarbonates can be continuously produced by a haloformation reaction in the presence of a molecular weight controlling agent.
For example, according to the process of British Patent No. 923,192, an aromatic dihydroxy compound, a molecular weight controlling agent, an aqueous solution of sodium hydroxide and phosgene are fed to the first reaction vessel in a series of four or five consecutively linked vessels and reacted. A polymerization catalyst is added to the second or the third reaction vessels where the reaction further proceeds, so as to carry out the polycondensation. As a result, an aromatic polycarbonate is produced.
However, when an aromatic polycarbonate is produced by the haloformation reaction in the presence of a molecular weight controlling agent, the rate of end capping of the resulting aromatic polycarbonate is as low as about 70-80%. In addition, according to the process of British Patent No. 923,192, the amount of hydrolysis of the carbonyl halide compound is not a negligible amount upon producing an aromatic polycarbonate.
On the other hand, it is also known to produce an aromatic polycarbonate in the presence of a polymerization catalyst.
For example, a liquid aqueous phase, a liquid organic phase and a gas phase containing phosgene are fed to a reactor containing packing in the presence of an amine or its salt. The reaction mixture passing through the final reactor is separated into a liquid phase and a gas phase, followed by isolating the reaction product from the liquid phase to obtain a high molecular weight linear polycarbonate [Japanese Patent Application Laid-open No. Sho 47-14297 (U.S. Pat. No. 3,787,359)].
As another example, a polycarbonate is prepared by a phase boundary condensation method comprising phosgenation of an aqueous alkali metal salt solution of one or more aromatic dihydroxy compounds in which process an aromatic chlorinated hydrocarbon is used as the solvent. The synthesis of the polycarbonate is carried out in two stages. In the first stage, the reaction of the alkali metal salt solution of the aromatic dihydroxy compound(s) with phosgene is carried out at an OH concentration of between 0.01 and 0.1% by weight of OH, relative to the aqueous phase, in the presence of 0.1 to 2.5 mole % of trialkylamine, relative to aromatic dihydroxy compound(s), and at a temperature higher than 70.degree. C. with a dwell time of less than 5 minutes. In the second stage, the polycondensation is effected by adjusting the OH concentration to 0.20 to 0.50% by weight of OH, relative to the aqueous phase, optionally with further addition of trialkylamine, at a temperature higher than 80.degree. C. and with a dwell time of more than 1 minute [Japanese Patent Application Laid-open No. Sho 50-122,595 (U.S. Pat. No. 4,038,252)].
According to the method of Japanese Patent Application Laid-open No. Sho 47-14297 (U.S. Pat. No. 3,787,359), there is used a reactor such as a packed column containing packing. Therefore, the stirring and mixing can not be efficiently effected as compared with the production of an aromatic polycarbonate using a tank reactor and the amount of hydrolyzed carbonyl halide compound increases disadvantageously.
According to the method of Japanese Patent Application Laid-open No. Sho 50-122,595 (U.S. Pat No. 4,038,252), the polymerization catalyst and the molecular weight controlling agent are added upon the haloformation reaction. Therefore, the rate of end capping is fairly improved and hydrolysis of the carbonyl halide compound is considerably suppressed.
The present inventors studied the above-mentioned method and found that when the aromatic dihydroxy compound was reacted with the carbonyl halide compound under conditions of low OH concentration in the aqueous phase in the presence of a trialkylamine, a decomposition reaction of the trialkylamine occurred, and the nitrogen content in the resulting aromatic polycarbonate increased. In general, such increased nitrogen content causes coloration of the aromatic polycarbonate upon high temperature shaping so that the aromatic polycarbonate thus shaped does not have a desirable color.
Therefore, there is desired a method for producing an aromatic polycarbonate having a low nitrogen content and a high rate of end capping while suppressing hydrolysis of a carbonyl halide compound.
The above-mentioned British Patent No. 923,192 discloses that polymerization catalyst is desirably added to the first tank reactor or the following tank reactor(s), preferably, to a later tank reactor where the reaction has fairly proceeded, but nothing is mentioned about the amount of the polymerization catalyst and the residence time in the tank reactor.
In the working example for producing an aromatic polycarbonate using a continuous tank reactor apparatus, the residence time in the first tank reactor is 4-5 min and a polymerization catalyst in an amount of 0.89 mole % based on the aromatic dihydroxy compound is added in the second or third tank reactor.
However, when the present inventors studied the example by adding the same amount of the polymerization catalyst to the first tank reactor, it was difficult to control the molecular weight and therefore, the resulting aromatic polycarbonate had a wide distribution of molecular weight.